Industrial furnaces lose 30 to 90% of the energy input in the form of hot flue gases. Heat exchangers can be employed to recover some of this energy in the form of preheat combustion air. Commercial heat exchangers are either of the recuperative or regenerative variety. The recuperative exchanger consists generally of tube and shell designed equipment, with the "coolant" being the combustion air. The equipment either can be located between the furnace and the stack, or can be an integral part of the stack. The heat transfer surface or tubes are constructed of either heat resisting metal alloy or ceramic. The regenerative heat exchanger cycles flue gases between two ceramic heat sinks (network of ceramic shapes which absorb heat, but permit the passage of the gas to the stack). While one ceramic heat sink is being contacted with hot flue gases, the other is being cooled by a flow of combustion air.
The application of conventional recuperative heat exchangers for high temperature furnaces (&gt;2000.degree. F.) has been hampered by high capital costs, space requirements and severe maintenance problems. In the latter regard, fouling of tubes and eventual tube "burn-out" has caused many installations to be abondoned and the furnaces are operating without heat recuperation.